Method for determination/compensation of bias errors/random walk errors induced by the light source in fiber-optic sagnac interferometers

ABSTRACT

A method for determination of/compensation for the bias/random walk errors induced by the light source in fiber-optic Sagnac interferometers employing a modulation method for stochastically independent shifting of the operating point to the points of highest sensitivity. A reference beam is output from the light beam emitted from the light source of the interferometer and passed to the fiber coil to produce a proportional reference intensity signal. Such signal is demodulated with the demodulation pattern of the rotation rate control loop to demodulate the rotation rate intensity signal (proportional to rotation rate). The demodulated reference intensity signal measures the bias/random walk errors to be determined. Demodulation of the reference intensity signal is simultaneous with that of the rotation rate intensity signal so that components of the reference and rotation rate intensity signals (each resulting from light components simultaneously emitted from the light source) are identically demodulated.

BACKGROUND

The present invention relates to fiber-optic Sagnac interferometers.More particularly, this invention pertains to a method for determinationof/compensation for bias/random walk errors induced by the light sourcein fiber-optic Sagnac interferometers.

DESCRIPTION OF THE PRIOR ART

Sagnac interferometers allow excellent measurement accuracy. Componentrequirements are correspondingly stringent. The light source is centralas light source noise can corrupt the rotation rate signal severely.Amplitude noise from the light source can cause errors known as “randomwalk”. Furthermore, bias errors can be produced by electricalinterference effects radiated into a light source supply voltage signal.

In order to avoid such errors, it is known to tap off a proportion ofthe light power in the form of a reference signal from the light beamemitted from the light source before it is fed into the fiber coil ofthe interferometer. This is converted to a corresponding intensitysignal by a monitor diode. The intensity signal can then be evaluated toreduce errors dependent upon the light source. One possibility is todemodulate the intensity signal supplied from the monitor diode and tosubtract the resulting error signal from the determined rotation ratesignal (compensation method). Another possibility, described, forexample, in U.S. Pat. No. 6,204,921, is to use the intensity signalproduced by the monitor diode as the controlled variable in a controlloop for controlling the light source current/the light sources. Theamplitude noise from the light source can be reduced by such means.However, only P regulators can be employed in this method as, although Iregulators result in the control error of the intensity signalintegrated over an infinitely long time tending to zero, a bias orrandom walk error from the gyro that tends to zero requires, incontrast, the integral of the intensity signal multiplied by thedemodulator (i.e. by the modulation signal) to be brought to zero. Thisrequirement cannot be easily satisfied by simple analog regulators.Further, such method can only be implemented with a continuousregulator.

SUMMARY AND OBJECTS OF THE INVENTION

It is therefore an object of the invention to provide a method ofgreater accuracy than conventional methods for determinationof/compensation for the bias/random walk errors induced by the lightsource in fiber-optic Sagnac interferometers.

The invention addresses the preceding and other objects by providing, ina first aspect, a method for determination of/compensation forbias/random walk errors induced by the light source in fiber-opticSagnac interferometers that use a modulation method for stochasticallyindependent shifting of the operating point to the points of highestsensitivity. Such method includes the step of outputting a referencebeam from the light passing through the interferometer using a couplerso that the intensity of the reference beam is proportional to theintensity of the light injected into the fiber optics of theinterferometer but not subject to any changes caused by modulationand/or resetting processes.

A reference intensity signal is produced that is proportional to theintensity of the reference beam. This is accomplished by tapping theoutput signal from a photodetector to which the reference beam isapplied.

The reference intensity signal is demodulated using the demodulationpattern used in the rotation rate control loop. Such signal is used fordemodulation of the rotation rate intensity signal (which isproportional to the rotational rate) using a demodulator. A demodulatedreference intensity signal is obtained by the demodulator thatrepresents a measure of the bias/random walk errors to be determined.The demodulation of the reference intensity signal is matched in time tothe demodulation of the rotation rate intensity signal. In this way, thesignal components of the reference intensity signal and the rotationrate intensity signal that result from light components simultaneouslyemitted from the light source are identically demodulated.

In another aspect, the invention provides a fiber-optic Sagnacinterferometer that employs a modulation method for stochasticallyindependent shifting the operating point to the points of highestsensitivity. Such interferometer includes a coupler for outputting areference beam from light emitted from the interferometer light sourceso that the intensity of the reference beam is proportional to that ofthe light injected into the fiber optics of the interferometer. It isnot subject to any changes caused by modulation and/or resettingprocesses.

A photodetector is provided. The reference beam is applied to suchphotodetector whose output signal is proportional to the intensity ofthe reference beam. A demodulator demodulates the reference intensitysignal using the demodulation pattern used in the rotation rate controlloop for demodulation of the rotation rate intensity signal. Such signalis proportional to rotation rate so that an output signal from thedemodulator represents a measure of the bias/random walk errors to bedetermined.

The demodulator is matched in time to demodulation of the rotation rateintensity signal so that the signal components of the referenceintensity signal and the rotation rate intensity signal (each of whichresult from light components simultaneously emitted from the lightsource) are identically demodulated.

The foregoing and other features of the invention will become furtherapparent from the detailed description that follows. Such description isaccompanied by a set of drawing figures. Numerals of the drawingfigures, corresponding to those of the written description, point tofeatures of the invention. Like numerals refer to like featuresthroughout both the written description and the drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a preferred embodiment of aninterferometer in accordance with the invention;

FIG. 2 illustrates a first preferred embodiment of a regulator inaccordance with the invention;

FIG. 3 illustrates a second preferred embodiment of a regulator inaccordance with the invention;

FIG. 4 illustrates a third preferred embodiment of a regulator inaccordance with the invention; and

FIG. 5 is a schematic diagram of a closed-loop fiber-optic Sagnacinterferometer in accordance with the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 5 is a schematic diagram of a closed-loop Sagnac interferometer inaccordance with the prior art. To assist an understanding of theinvention, the following text briefly describes the method of operationof a closed-loop Sagnac interferometer that employs a modulation methodfor stochastically independent shifting of the interferometercharacteristic or of the operating point of the interferometer to thepoints of highest sensitivity.

Referring to FIG. 5, the light from a light source 1 of stabilizedintensity and wavelength is passed via a fiber path to a first beamsplitter 2, and, from there via a polarizer 3, to a second beam splitter4. The beam elements produced by splitting are passed from the twooutputs/inputs remote from the light source 1 to the two inputs/outputsof a fiber coil 5. A phase modulator 6 is arranged between theoutputs/inputs of the second beam splitter 4 and the inputs/outputs ofthe fiber coil 5. The beam elements, which interfere in the second beamsplitter 4 after passing through the fiber coil 5, once again passthrough the polarizer 3, and as great a proportion as possible is passedvia the first beam splitter 2 to a photodetector 7. The output signalfrom the photodetector 7 is first amplified by an amplifier 8, thenapplied to a demodulator 9 and a synchronous demodulator 10. Togetherwith an amplification filter 11, the demodulator 9 forms a scale factorcontrol path. The synchronous demodulator 10 employs a filter 12 todrive a ramp generator 13 that is used to produce a reset signal. Thesignal produced by a modulation oscillator 14 to shift the operatingpoint to that of highest sensitivity and the reset signal are combinedat an adder 15 to form a single signal. Such signal is the input to acontrollable amplifier 16 that amplifies it by an output signal from theamplification filter 11 that is used as a gain factor. The output signalfrom the controllable amplifier 16 is, in turn, used to drive the phasemodulator 6.

The interferometer according to the invention will now be described inmore detail with reference to FIG. 1, a schematic description of apreferred embodiment of an interferometer in accordance with theinvention. Components and devices that correspond to those shown in FIG.5 are identified by the same reference numerals.

The light produced by a light source 1 is passed via a first coupler 2,a spatial filter 3 and a second coupler 17 to a main beam splitter 4.The beam elements produced at the main beam splitter 4 pass through afiber coil 5 and are combined to form an interference beam. Suchinterference beam is passed via the second coupler 17 and the spatialfilter 3 to the first coupler 2 that outputs a specific proportion ofthe interference beam and passes it to a rotation rate detector 7. Anoutput signal from the rotation rate detector 7 (an intensity signalproportional to the rotation rate) is applied to a rotation rateregulator 13 that controls a phase modulator (not shown) so that themeasured rotation rate disappears. (The components for stochasticallyindependent shifting of the interferometer characteristic, or operatingpoint, of the interferometer to the points of highest sensitivity arenot shown in FIG. 1 for clarity.)

The interferometer of FIG. 1 essentially differs from that of FIG. 5 bythe addition of a control loop for controlling light source power. Suchcontrol loop includes the second coupler 17, a light power detector 18and a light power regulator 19. The second coupler 17 outputs areference beam taken from the light beam coming from the light sourceand passing toward the fiber coil 5. This is supplied to the light powerdetector 18. An output signal from the light power detector 18, σ(ν),proportional to the intensity of the reference beam, is applied to thelight power regulator 19 that controls light source power as a functionof it to compensate for bias/random walk errors. (The expression “fiberoptics in the interferometer” in the exemplary embodiment illustrated inFIG. 1 refers in particular to the components annotated with thereference numbers 1, 2, 3, 4, 5 and 17 and to the fiber paths betweenthem.)

FIG. 2 illustrates a first preferred embodiment 20 of the light powerregulator 19 in accordance with the invention. A time-discrete method isdescribed in conjunction with such embodiment that can be carried outdigitally. A continuous (analog) version of such method is alsopossible.

The signal σ(ν) produced and sampled by the light power detector 18(preferably in synchronism with the rotation rate detector) isdemodulated in a demodulator 21 (i.e. multiplied by the demodulationsignal m(ν+1)). The demodulation signal m(ν)ε{−1,1} is the samedemodulation signal as that employed in the rotation rate control loop(i.e. the random demodulation signal from the main control loop, thedemodulation signal used by the demodulators with the reference numbers9 and 10 in FIG. 5 produced by the modulation oscillator 14). In theory,the demodulation signal is the same as that of the main control loop.

The choice of demodulation signal is open. It is possible, for example,to use deterministic modulation/demodulation. In such case, theparameter ν would represent a discrete time-dependent value. Sincesignal components output at the same time from the light source 1 arriveat the rotation rate detector 7 and the light power detector 18 atdifferent times, the demodulation signal and the demodulation patternm(ν) must be made available to the two detectors at different times. Insuch an embodiment, the respective fiber paths to the detectors 7 and 18are such that a relay time difference of one working clock cycle of theinterferometer is created. The light power detector 18 is thereforeprovided with the demodulation signal m(ν) with a negative delay(“leading” by one working clock cycle) as this detector receives thelight one working clock cycle earlier than the rotation rate detector 7.The demodulated signal (i.e. the signal multiplied by m(ν+1)) is passedvia an adder stage 22 to an accumulator 23 that integrates the inputsignal and supplies the integrated output signal to a modulator 24. Suchmodulator 24 multiplies the integrated signal by the demodulation signalm(ν+1) and produces a corresponding output signal clq(ν) for driving thelight source 1. The integrated signal modulated with m(ν+1) is appliedto the light source current (with a negative mathematical sign) by suchmeans. This insures that the different gradients of the intensity curveat the points of highest sensitivity are taken into account (i.e.long-term correlations of σ(ν) with m(ν+1) disappear). Long-termcorrelations of the light source noise with the demodulation signal ofthe rotation rate regulator 7 are thus eliminated.

Should the five paths from the light source 1 to the rotation ratedetector 7 and the light power detector 18 be of equal lengths (e.g. byprovision of an additional fiber coil in the fiber path between thesecond coupler 17 and the light power detector 18), there is no need toshift the demodulation patterns by one working clock cycle. All that isthen required is to insure that signal components in the output signalfrom the rotation rate detector 7 and from the light power detector 18that result from light components emitted simultaneously from the lightsource 1 are identically demodulated.

FIG. 3 illustrates a second preferred embodiment of a regulator 30 inaccordance with the invention. The light power regulator 30 supplies theoutput signal from the light power detector 18 to an accumulator 32 viaan adder stage 31. An integrated output signal from the accumulator 32is supplied to a demodulator 33 that demodulates the signal with asignal Δm(ν+1)=m(ν+1)·m(ν). The regulators of FIGS. 2 and 3 areinterchangeable, producing the same output signal for the same inputsignal.

The method described above is time-discrete and can be implemented inthe digital electronics of the interferometer. Such method takes accountof the given demodulation pattern and eliminates long-term correlationsbetween light source noise and the demodulation signal from the main(rotation rate) regulator. It is based on an additional regulator (thelight power regulator) that receives its input from a monitor photodiodeand whose output modulates the light source 1 power such that “rotationrate components” disappear from the light signal.

The method described above can also be implemented in analog form. FIG.4 illustrates a third preferred embodiment, an analog regulator 40, usedfor this purpose. It supplies the output signal from the light powerdetector 18 to an analog multiplier 41 where it is multiplied by thedemodulation signal m(ν+1) from the main control loop. The demodulatedsignal is supplied to an analog integrator 42. The output of the analogintegrator 42 is supplied to an analog multiplier 43 that multipliesthis signal by the demodulation signal m(ν+1). The corresponding outputsignal clq(ν) controls light source current.

One idea on which the invention is based is that, when using amodulation method for stochastically independent shifting of theoperating point to the points of highest sensitivity, the demodulationpattern of the rotation rate control loop should be taken into accountin the determination of bias/random walk errors which are specific tothe light source. This is because the gradients of the intensity curveat the points of highest sensitivity have different mathematical signs,which lead to “incorrect” intensity signals if they are not taken intoaccount.

As mentioned above, demodulation of the reference intensity signalpreferably leads demodulation of the rotation rate intensity signal byone interferometer working clock cycle. The expression “working clockcycle” refers to the time for the light to pass through the fiber coil.

To compensate for determined bias/random walk errors, a correspondinglight source drive signal is produced and light source power iscontrolled on the basis of the demodulated reference intensity signalsuch that the determined bias/random walk errors are compensated.

As already mentioned, the reference beam can be output at any desiredpoint between the light source and the fiber coil of the interferometer.In this case, the following should be taken into account: the referencebeam can in principle be tapped off at any point where its intensity isproportional to the intensity that is stored in the optics of the FOGand is still independent of the modulation and resetting signals thatare applied by virtue of the operation of the FOG. If required, thissignal can also be tapped off from a monitor detector located directlyadjacent the light source. In principle, however, it must be borne inmind that the light from the light source is composed directly after itenters the fiber of more polarization states and modes than is the case,finally, at the end of the overall optical path (i.e. directly upstreamof the photodetector of the rotation rate regulator) since the pathcontains mode and polarization filters. It is of critical importancethat the monitor detector “see” precisely those modes and polarizationstates that the detector for the rotation rate regulator also sees. Werethe monitor detector to be supplied with further modes and polarizationstates, then, on average, it would regulate the demodulated sum of allof such components to zero. The effective state/mode would then have tocompensate for the ineffective states/modes. This would result in anincreased rotation rate error/random walk in the rotation rate controlloop, which reacts only to the effective state/mode. This would, ofcourse, be counter-productive for the purposes of the idea according tothe invention.

The output signal from the demodulator is a measure of the bias/randomwalk errors to be determined. The demodulator is matched in time to thedemodulation of the rotation rate intensity signal so that the signalcomponents of the reference intensity signal and the rotation rateintensity signal (which each result from light components emitted fromthe light source at the same time) are demodulated in an identicalmanner. To achieve this, it is possible, for example, to relate the pathlengths the light travels from the light source to the photodetector toproduce the rotation rate intensity signal (proportional to rotationrate) and to produce the reference intensity signal to one another suchthat they differ by the path length which the light travels during oneinterferometer working clock cycle. The demodulation patterns (which areidentical) are accordingly then shifted through one interferometerworking clock cycle with respect to one another.

The interferometer preferably has a regulator which controls the lightsource power on the basis of the output signal from the demodulator,such that the determined bias/random walk errors are compensated for.Together with the demodulator and the photodetector, the regulator formsa corresponding control path.

A method and interferometer according to the invention offers thefollowing features in contrast to the prior art: 1) suitability forfiber gyros with random modulation as it includes the randommodulation/demodulation pattern (the random modulation method isexplained in greater detail, for example, in European patentspecification EP 0 551 537 B1); 2) implementation of an integrating,optionally analog or discrete, control loop including a monitor diodeand a light-source intensity modulator to nullify interference and noisesignals in the light source signal that otherwise lead to long-termerrors (random walk and bias) in the gyro signal; and 3) the rotationrate detector and the light power detector can be designed identically(including sampling times, optimal filters and blanking), to obtainmaximum compensation.

While this invention has been disclosed with reference to itspresently-preferred embodiment, it is not limited thereto. Rather, theinvention is limited only insofar as it is defined by the following setof patent claims and includes within its scope all equivalents thereof.

1. A method for determination of/compensation for bias/random walkerrors induced by the light source in fiber-optic Sagnac interferometerswhich use a modulation method for stochastically independent shifting ofthe operating point to the points of highest sensitivity, comprising thefollowing steps: outputting of a reference beam from the light passingthrough the interferometer using a coupler, in such a way that theintensity of the reference beam is proportional to the intensity of thelight injected into the fiber optics of the interferometer, but is notsubject to any changes caused by modulation and/or resetting processes:production of a reference intensity signal which is proportional to theintensity of the reference beam by tapping the output signal from aphotodetector to which the reference beam is applied; demodulation ofthe reference intensity signal using the demodulation pattern which isused in the rotation rate control loop and is used for demodulation ofthe rotation rate intensity signal, which is proportional to therotational rate, using a demodulator, by which means a demodulatedreference intensity signal is obtained which represents a measure of thebias/random walk errors to be determined, with the demodulation of thereference intensity signal being matched in time to the demodulation ofthe rotation rate intensity signal such that the signal components ofthe reference intensity signal and of the rotation rate intensity signalwhich each result from light components emitted from the light source atthe same time are demodulated in an identical manner.
 2. The method asclaimed in claim 1, characterized in that the demodulation of thereference intensity signal precedes the demodulation of the rotationrate intensity signal by one interferometer working clock cycle.
 3. Themethod as claimed in claim 1, characterized in that the light sourcepower is controlled on the basis of the demodulated reference intensitysignal such that the determined bias/random walk errors are compensatedfor.
 4. A fiber-optic Sagnac interferometer which uses a modulationmethod for stochastically independent shifting of the operating point tothe points of highest sensitivity, having: a coupler for outputting areference beam from light which is emitted from the light source of theinterferometer, in such a way that the intensity of the reference beamis proportional to the intensity of the light injected into the fiberoptics of the interferometer, but is not subject to any changes causedby modulation and/or resetting processes; a photodetector, to which thereference beam is applied and whose output signal is proportional to theintensity of the reference beam, a demodulator for demodulation of thereference intensity signal using the demodulation pattern which is usedin the rotation rate control loop and is used for demodulation of therotation rate intensity signal, which is proportional to the rotationrate, such that an output signal from the demodulator represents ameasure of the bias/random walk errors to be determined, with thedemodulator being matched in time to the demodulation of the rotationrate intensity signal such that the signal components of the referenceintensity signal and of the rotation rate intensity signal which eachresult from light components emitted from the light source at the sametime are demodulated in an identical manner.
 5. The interferometer asclaimed in claim 4, characterized in that the path lengths which thelight must travel from the light source to the photodetector in order toproduce the rotation rate intensity signal which is proportional to therotation rate, and to the photodetector in order to produce thereference intensity signal are related to one another in such a way thatthey differ by the path length which the light travels in oneinterferometer working clock cycle, and the demodulation patterns areshifted in a corresponding manner with respect to one another by oneinterferometer working clock cycle.
 6. The interferometer as claimed inclaim 4, characterized by a regulator, which controls the light sourcepower on the basis of the output signal from the demodulator such thatthe determined bias/random walk errors are compensated for.